Method and apparatus for producing a welded joint in thermoplastic strap with differential pressure

Abstract

A novel method and apparatus are provided for making an improved welded joint between overlapping lengths of thermoplastic strap. The welded joint includes a central region and an adjacent peripheral region. The pressure on the strap joint during formation is released first in the peripheral region and subsequently in the central region. An embodiment of the apparatus for making such a joint includes an anvil, a central strap-engaging member, a peripheral strap engaging member, and a mechanism for moving the anvil sequentially to three positions. At a first position the overlapping strap lengths are pressed between the anvil and both the central and peripheral strap-engaging members. In the second position of the anvil the peripheral strap-engaging member does not press against the strap lengths while the central strap-engaging member continues to press the strap lengths against the anvil. In the third position of the anvil the anvil and strap-engaging members are sufficiently spaced apart to release all of the pressure on the strap lengths.

What is claimed is: 
     
       1. Apparatus for making a welded joint between first and second overlapping lengths of flexible, thermoplastic strap, said apparatus comprising: a frame;   an anvil supported on said frame for movement to a position in which it can be engaged by said first overlapping length of strap;   a drive shaft mounted for rotation in said frame about an axis and a cylindrical drive cavity defined in said drive shaft with the cavity axis being parallel to, but offset radially from, the drive shaft axis;   a peripheral strap-engaging member having a cylindrical driven shaft rotatably mounted in said drive shaft cavity and having an annular strap-contacting surface engageable with said second overlapping length of strap; a central strap-engaging member carried by said peripheral strap-engaging member and being engageable with said second overlapping length of strap within said annular strap-contacting surface of said peripheral strap-engaging member;   a resilient means carried by said peripheral strap-engaging member for biasing said central strap-engaging member relative to said peripheral strap-engaging member toward said anvil from a fully depressed position to fully extended position;   positioning means on said frame for moving said anvil to (1) a first position in which said anvil can be engaged by said first overlapping length of strap and in which both said central and peripheral strap engaging members engage said second overlapping length of strap to force said lengths of strap into face-to-face contact under pressure, (2) a second position in which said anvil and said peripheral strap-engaging member are sufficiently spaced apart to release the pressure of said peripheral strap-engaging member on said lengths of strap while still maintaining the pressure of said central strap engaging member on said lengths of strap, and (3) a third position in which said anvil and said central strap-engaging member are sufficiently spaced a part to release the pressure of said central strap-engaging member on said lengths of strap; and   means for increasing the temperature of the contacting faces of the overlapping strap lengths to melt at least part of the thickness of each said strap including rotating means for rotating said drive shaft about said shaft axis in the first position of said anvil whereby both of said central and peripheral strap-engaging members are oscillated in a generally circular orbit about said shaft axis causing relative generally circular orbital slidable frictional movement between said contacting faces of said overlapping strap lengths.   
     
     
       2. Apparatus for making a welded joint between first and second overlapping lengths of flexible, thermoplastic strap, said apparatus comprising: an anvil for engaging said first overlapping length of strap;   a central strap-engaging member for engaging said second overlapping length of strap;   a peripheral strap-engaging member for engaging said second overlapping length of strap around said central strap- engaging member;   pressing means for effecting relative movement between said anvil on one hand and each of said strap-engaging members on the other hand to (1) a first position in which said anvil engages said first overlapping length of strap and both said central and peripheral strap-engaging members engage said second overlapping length of strap to force said lengths of strap into face-to-face contact under pressure, (2) a second position in which said anvil and said peripheral strap-engaging member are sufficiently spaced apart to release the pressure of said peripheral strap-engaging member on said lengths of strap while still maintaining the pressure of said central strap engaging member on said lengths of strap, and (3) a third position in which said anvil and said central strap-engaging member are sufficiently spaced apart to release the pressure of said central strap-engaging member on said lengths of strap; and   means for increasing the temperature of the contacting faces of said strap lengths to melt at least part of the thickness of each said strap length.   
     
     
       3. The apparatus in accordance with claim 2 in which said pressing means includes (a) resilient means for biasing said central strap-engaging member relative to said peripheral strap-engaging member toward said anvil from a fully depressed position to a fully extended position, and   (b) positioning means for (1) moving said anvil in a first direction to squeeze said overlapping lengths of strap against both said central and peripheral strap-engaging members at said first position,   (2) moving said anvil in a second direction opposite said first direction to said second position in which said central strap-engaging member remains biased in engagement with said second overlapping length of strap and said peripheral strap-engaging member is disengaged from said second overlapping length of strap, and   (3) moving said anvil further in said second direction to said third position to release the pressure on said overlapping lengths of strap between said anvil and said central strap-engaging member.     
     
     
       4. The apparatus in accordance with claim 2 in which said peripheral strap-engaging member is toothed for gripping said second overlapping length of strap. 
     
     
       5. The apparatus in accordance with claim 2 in which said anvil has a toothed, strap-contacting surface and said central strap-engaging member has a generally circular, smooth, strap-contacting surface to accommodate slipping relative to the face of said second overlapping length of strap. 
     
     
       6. The apparatus in accordance with claim 2 in which said means for increasing the temperature includes an oscillating means for effecting a relative closed path oscillating movement between said anvil on the one hand and both of said strap-engaging members on the other hand to effect relative bodily sliding movement between said strap lengths. 
     
     
       7. The apparatus in accordance with claim 6 in which said central strap-engaging member is carried by said peripheral strap-engaging member;   said apparatus includes a frame;   said oscillating means includes a drive shaft mounted for rotation about an axis in said frame and a cylindrical drive cavity defined in said drive shaft with the cavity axis being parallel to, but offset radially from, the drive shaft axis; and   said peripheral strap-engaging member includes a cylindrical driven shaft rotatably mounted in said drive shaft cavity.   
     
     
       8. The apparatus in accordance with claim 7 in which said peripheral strap-engaging member defines a bore surrounded by an annular, toothed, strap-contacting surface;   said central strap-engaging member is slidable disposed in said recess for movement between a fully depressed position and a fully extended position; and   said pressing means includes (a) a compression spring mounted in said bore and engaged on one end with said peripheral strap-engaging member and on the other end with said central strap-engaging member to bias said central strap-engaging member toward said fully extended position, and   (b) positioning means for (1) moving said anvil in a first direction to squeeze said overlapping lengths of strap against both said central and peripheral strap-engaging members at said first position,   (2) moving said anvil in a second direction opposite said first direction to said second position in which said central strap-engaging member remains biased in engagement with said second overlapping length of strap and said peripheral strap-engaging member is disengaged from said second overlapping length of strap, and   (3) moving said anvil further in said second direction to said third position to release the pressure on said overlapping lengths of strap between said anvil and said central strap-engaging member.       
     
     
       9. Apparatus for making a welded joint between first and second overlapping lengths of thermoplastic strap, said apparatus method comprising: pressing means for forcing said strap lengths into face-to-face contact at above-ambient pressure along an interface in a central region and in an adjacent peripheral region;   means for increasing the temperature of said strap lengths along said interface sufficiently to melt at least part of the thickness of each strap portion at said interface; and   means for releasing said above-ambient pressure first in said peripheral region and subsequently in said central region whereby said melted parts of said strap lengths along said interface solidify to form said welded joint.   
     
     
       10. The apparatus in accordance with claim 9 in which said means for increasing the temperature includes means for effecting relative bodily sliding movement between said strap lengths whereby at least part of the thickness of each said strap length melts along said interface in said central and peripheral regions. 
     
     
       11. The apparatus in accordance with claim 9 in which said apparatus includes an anvil, a peripheral strap-engaging member with an annular strap-contacting surface, and a central strap-engaging member with a central, circular, strap-contacting surface disposed within said annular strap-contacting surface.